An electrochemical cell is the basic building block for a battery, used for storing electricity.

The picture above shows two cells (the white cylinders on the left) and a cutaway battery.

Each cell has a positive electrode and a negative electrode, which are immersed in a liquid or gel electrolyte and separated by a membrane. Positive ions can pass through the membrane but electrons cannot, and are forced to travel around the electric circuit instead.

Several different chemistries are in use. Lead-Acid cells are commonly used in the batteries fitted to conventional cars for starting and running electrical equipment. Electric vehicles commonly use lithium ion cells, though now nickel cobalt manganese and nickel cobalt aluminium cells are being introduced. These offer greater power density, but at greater cost.